The present study is designed to provide information concerning the basic genetics of Aspergillus flavus and evidence for the genetic control of aflatoxin synthesis. This is to be accomplished by: (1) the induction and recovery of auxotrophs and aflatoxin variants, (2) the assignment of genes controlling aflatoxin production to specific linkage groups, (3) the determination of linkage of aflatoxin genes to known genetic markers, (4) the analysis of gene action in the control of different aflatoxin levels, and (5) the examination of the relationship between aflatoxins B1 and B2 from studies of haploids, heterozygous diploids, and haploid segregants derived from these diploids. Eight linkage groups have been identified in A. flavus by means of the parasexual cycle and 23 gene loci have been assigned to these linkage groups. A mutant producing a high and relatively stable aflatoxin B2:B1 ratio was recovered. Parasexual crosses involving the mutant have revealed a genetic locus for control of aflatoxin synthesis. Additional aflatoxin variants have been recovered and some have been examined in heterozygous diploids. Diploids, on the average, produced twice as much aflatoxin as haploids. Although most gene effects appeared to be additive, non-additive and dominance effects have also been noted. BIBLIOGRAPHIC REFERENCES: Papa, K.E. 1977. Mutant of Aspergillus flavus producing more aflatoxin B2 than B1. Appl. Env. Microbiol. 33, in press. Papa, K.E. 1977. Genetic analysis of a mutant of Aspergillus flavus producing more aflatoxin B2 than B1. Mycologia 69, in press.